UPDATE: IPv6 load balancer endpoints are now GA.1 However, we continue to work on instance-level v6 addressing. Since I believe that at least some votes on this issue are for v6 all the way to the instance, I am leaving this issue open.Sean J. Schluntz commented
Despite the low adoption of IPv6 worldwide it is very deeply integrated into many market segments (like education). Not being able to use IPv6 within GCE creates limitations on what can and can't be pushed out into the cloud. Adding support for it, even in limited ways (like in only one zone to start) would go a long way to helping adoption.